Organic acids are widely used in various industrial fields including foods, medicines and pharmaceuticals, cosmetics, etc. Since the first commercialization of lactic acid in 1880, citric acid was commercialized in 1923, and the commercialization of acetic acid, itaconic acid, succinic acid, etc. followed. These organic acids are mostly produced by fermentation or chemical synthesis, and among them, 70 or more organic acids are produced by fermentation and are synthesized via glycolytic and citric acid pathways. Among these organic acids, acetic acid, lactic acid, citric acid, tartaric acid, malic acid, etc. can be produced on a large scale. In particular, in the case of lactic acid, polylactic acid has been confirmed to be a biodegradable polymer that can replace plastics produced from petroleum, and thus there has been a significant increase in the demand for lactic acid used as a monomer.
For the use of lactic acid as a monomer of polylactic acid, it is necessary to selectively produce optical isomers of lactic acid, and in this regard, a method of using a microorganism is preferred to the existing method of chemical synthesis. There are many known Lactobacilli, which are microorganisms producing lactic acid. However, when lactic acid is produced by culturing these microorganisms, there is a disadvantage in that two kinds of optical isomers (D-lactate and L-lactate) are produced together, thus requiring genetic manipulation, and also the microorganisms have a weak acid resistance.
Additionally, the lactic acid accumulated during the fermentation process of lactic acid can acidify the pH of a given medium, thus inhibiting the growth of a given strain. Therefore, a method of adding a neutralizing agent to the medium has been used in the lactic acid fermentation. Calcium carbonate, which is most widely used in the above method, can form a calcium salt by binding to lactic acid, and during the process of recovering lactic acid by treating precipitates with an acid solution after the fermentation process, lime can be formed in the same molar amount as that of lactic acid. The method can complicate the purification process, generate strong acidic waste water, and form by-products, thereby increasing the production cost of lactic acid. To solve these problems, there is a growing interest on the use of acid-resistant microorganisms capable of minimizing the use of a neutralizing agent (Biotechnolbi Advances, 2013; 31: 877-902). Although wild-type yeasts cannot produce lactic acid, they have a significantly higher stress resistance than that of bacteria, and thus studies on lactic acid production in yeasts based on this characteristic are competitively conducted. In particular, studies on lactic acid production in an acidic condition without using a neutralizing agent were conducted in various kinds of yeasts by manipulating the ethanol production pathway of an acid-resistant strain to produce lactic acid instead of ethanol (Biotechnol Genet Eng Rev. 2010; 27: 229-256).
Additionally, bioethanol, which has a high potential as a green fuel, is utilized as an alternative fuel that can simultaneously solve serious environmental problems such as global warming while reducing dependence on oil energy. However, the rapidly-growing demand for the first generation bioethanol has caused the prices of crop products such as corn and sugarcane to soar, and in addition, has induced the rise of the prices of other crops, and thus the bioethanol production for the resolution of energy issues is becoming a new cause for food problems.
Due to the above problem, the so-called second generation biofuel, cellulosic bioethanol, is spotlighted as a new alternative. Cellulosic bioethanol is ethanol prepared from glucose, which is produced by decomposing cellulose, the most abundant organic material on earth and a major constituting component of plant cell walls. Accordingly, unlike the first generation method of producing ethanol by fermentation of corn starch at present, the method for producing cellulosic bioethanol is a method for producing bioethanol using all tissues such as leaves, stems, roots, etc. of corn, which have been discarded or burnt. By the above method, bioethanol can be produced from the tissues of all plants such as corn husk, rice bran, grass, reeds, flame grass, wood waste, etc.
The materials serving as raw materials for the second generation biofuel are called lignocellulosic biomass, and lignocellulosic biomass largely consists of three components: cellulose, hemicelluloses, and lignin. Among these, hemicelluloses contain arabinose and xylose in an amount of about 5% to about 20%. The production of bioethanol using pentose sugars, which constitute a significant part of hemicelluloses, is currently performed with many limitations.
Additionally, research on biodiesel, which is an environmentally friendly clean alternative energy, has been actively conducted. However, biodiesel has a disadvantage in that it produces glycerol as a by-product. Glycerol can be used as a raw material for cosmetics, food preparations, and petrochemical derivatives but it has a disadvantage in that there is a limitation on host cells that can utilize it as a raw material for sugar.